Method of making a lithium-silicon boron electrode for electrical energy storage device

ABSTRACT

A method for forming an improved lithium electrode structure which comprises an alloy of lithium, silicon, and boron in specified proportions and a supporting current-collecting matrix in intimate contact with said alloy. The lithium formed by the method of the present invention is utilized as the negative electrode in a rechargeable electrochemical cell.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 793,815, filedMay 5, 1977 now U.S. Pat. No. 4,076,905.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention broadly relates to forming a lithium electrode structureand a secondary electrochemical cell utilizing such an electrode.

2. Prior Art

Two approaches generally have been followed in the construction of alithium electrode for use in an electrical energy storage device, suchas a rechargeable battery, particularly one employing a molten saltelectrolyte. In one approach, the lithium is alloyed with another metalsuch as, for example, aluminum to form a solid electrode at theoperating temperature of the cell. In the other approach, liquid lithiumis retained in a foraminous metal substrate by capillary action.Heretofore, the latter approach has been preferred because it providescells with higher operating cell voltages and therefore potentiallyhigher battery energy densities. Certain problems are encountered,however, when it is attempted to retain molten lithium in a foraminousmetal substrate. More particularly, most metals which are readily wettedby lithium are too soluble in the lithium to permit their use as themetal substrate, whereas most metals structurally resistant to attack bymolten lithium are poorly wetted by the lithium when placed in a moltensalt electrolyte.

It has been suggested that metals structurally resistant to attack bymolten lithium may be wetted by immersion in molten lithium maintainedat a high temperature. However, the structure so wetted by lithium atthese higher temperatures usually undergoes progressive de-wetting whenused as the negative electrode in a secondary battery containing amolten salt electrolyte maintained at the substantially lowertemperatures at which such a battery operates. Thus after operation ofthe battery for a number of cycles, it has been found that lithium nolonger preferentially wets the substrate, the electrode progressivelylosing capacity. Various methods have been proposed in an attempt toovercome this problem. See, for example, U.S. Pat. Nos. 3,409,465 and3,634,144. None of the proposed methods have proven entirelysatisfactory.

The use of a solid lithium alloy as taught by the prior art also is notwithout problems. More particularly, lithium-aluminum alloy, forexample, is approximately 300 millivolts more positive than liquidlithium. Thus, electrochemical cells utilizing lithium-aluminum alloysas electrodes are not able to achieve the same potentials as thoseutilizing liquid lithium electrodes. Further, in a molten saltelectrolyte, the lithium-aluminum alloy electrode expands and contractsgreatly during charging and discharging of the electrochemical cell.Thus, it has been reported that the lithium-aluminum electrode maychange in volume by as much as 200% during charging and discharging ofthe cell. Still further, lithium-aluminum alloys generally are limitedto a lithium content of less than about 30 wt.%.

Various other materials have been suggested for use as an alloy withlithium to form a solid electrode. In U.S. Pat. No. 3,506,490, forexample, it is suggested that the lithium be alloyed with eitheraluminum, indium, tin, lead, silver, or copper. However, none of thesematerials have been proven to be completely satisfactory. Moreparticularly, these other suggested materials, such as tin and lead forexample, form alloys containing lesser amounts of lithium than doesaluminum, and thus have a still lower capacity (ampere-hours) per unitweight of alloy. Further, the potential of these other alloys comparedwith liquid lithium is more positive than that of a lithium-aluminumalloy; thus, alloys of such other materials are less desirable. Otherpatents relating to solid lithium anodes include U.S. Pat. Nos.3,506,492 and 3,508,967.

As a means of resolving some of the foregoing problems, U.S. Pat. No.3,969,139 provides an electrode structure utilizing an alloy of lithiumand silicon, this electrode being of particular utility as the negativeelectrode in a rechargeable lithium-metal sulfide molten salt cell. Suchan electrode provides excellent lithium retention, significantly reducescorrosion, and provides twice the energy capacity of thelithium-aluminum electrode.

However, it was subsequently found that in electrochemically forming thelithium-silicon alloy electrode, not all the silicon was utilizable inthe electrochemical forming process, thereby requiring a greater amountof silicon for a given ampere-hour capacity. Also, when utilized at highcurrent densities, the lithium-silicon alloy electrode tended to becomepolarized during electrochemical transfer of lithium into and out of theelectrode.

Co-pending U.S. patent application Ser. No. 715,358, filed Aug. 18, 1976now U.S. Pat. No. 4,048,395, and assigned to the Assignee of the presentinvention, suggests that the foregoing problems are substantiallyovercome through the use of a ternary alloy of lithium, silicon andiron. The use of a lithium-silicon-iron alloy permits substantiallycomplete utilization of the silicon and further reduces the tendency ofthe electrode to polarize at high current densities. However, anotherproblem has been found with the use of lithium-silicon alloys which isnot overcome by the addition thereto of iron. Specifically, it has beenfound that during cycling of a lithium-silicon electrode the silicontends to migrate into the metal substrate supporting structure causingthe metal to become brittle, lose its structural integrity and graduallydisintegrate into small particles. Accordingly, the need still existsfor an improved lithium electrode which retains the advantageousfeatures of the lithium-silicon alloy electrode while at the same timeminimizing or eliminating the disadvantageous features thereof.

SUMMARY OF THE INVENTION

Broadly, the present invention provides a method for forming an improvedlithium electrode structure, compared with the lithium-aluminum,lithium-silicon, and lithium-silicon-iron alloy electrodes, for use inan electrical energy storage device such as a secondary battery orrechargeable electrochemical cell. The improved electrode structureformed by the method of the present invention comprises a ternary alloyof lithium, silicon, and boron in intimate contact with a supportingcurrent-collecting matrix. The formed or fully charged alloy may berepresented by the empirical formula Li_(x) SiB_(y) where x may have anyvalue from 1 to 5 and y may have any value from 0.1 to 3. For preferredalloy compositions, x has a value from 4 to 5 and B has a value from 0.2to 1.0, all ranges stated being inclusive.

The ternary alloy electrode is electrochemically formed starting with amixture of boron and silicon, substantially complete utilization of thesilicon present being obtained during the forming process. Thereby lesssilicon will be required to obtain an electrode having a givenampere-hour capacity compared with electrochemical forming when startingwith silicon metal alone. In addition, boron substantially inhibits themigration of silicon into the electrode substrate or supporting matrix,thus, the use of boron permits the use of cheaper substrate or matrixmaterials such as iron and stainless steels without the resultingembrittlement that would occur using lithium-silicon alone or a ternaryalloy of lithium-silicon-iron as disclosed in U.S. Ser. No. 715,358filed Aug. 18, 1976. Another advantage of boron over iron is that theternary boron alloy occupies less space thus permitting a higher loadingof lithium per unit volume. Still further the ternary boron alloys ofthe present invention are lighter than the ternary iron alloys of theaforesaid patent containing an equivalent amount of lithium. Weight isof particular importance for batteries for use in electric vehicles,thus the present invention is of particular utility in providingelectrodes for such use.

The presence of boron facilitates the electrochemical utilization of thesilicon and substantially reduces the migration of silicon into theelectrode substrate. At the same time, however, it is electrochemicallyinert with respect to the cell process, so it will be appreciated thatthe amount of boron present in the formed electrode, as well as that ofsilicon, will be kept to a minimal value consistent with that requiredfor obtaining the improved advantageous features of the present lithiumalloy electrode.

The improved electrical energy storage device utilizing this electrodepreferably comprises a rechargeable lithium battery having positive andnegative electrodes spaced apart from one another and in contact with asuitable lithium-ion-containing electrolyte, preferably a molten saltelectrolyte. The improved lithium electrode formed by the method of thepresent invention is utilized as the negative electrode, functioning asthe cell anode during the discharge mode of the cell.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1A and 1B show on a triangular coordinate scale the Li-Si-B alloysystem formed by the method of the present invention;

FIG. 2 is a graphical representation of a typical charge-discharge curvecharacteristic of the electrode formed by the method of the presentinvention;

FIG. 3 is a pictorial view in perspective of an electrode formed by themethod of the present invention; and

FIG. 4 is a diagrammatic representation of an electrical energy storagedevice utilizing the electrode formed by the method of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The lithium electrode structure formed by the method of the presentinvention in its broadest aspects comprises a ternary alloy of lithium,silicon, and boron in intimate contact with a supportingcurrent-collecting matrix, thereby generally providing a unitary orcomposite electrode structure. The term "alloy" as used herein isdefined as an intimate mixture of the three metals in which the metalsmay form mixed crystals, solid solutions, or chemical compounds. Themetals also may be present in more than one of these states in the samealloy. For convenience in characterizing and describing the ternaryalloy, reference will be made to the composition of the alloy based onweight percentages, atom percentages, and exemplary empirical formulas.

In Table 1 five alloy compositions are shown, based on the empiricalformula Li_(x) SiB_(y), where x may have any value from 1 to 5 and y mayhave any value from 0.1 to 3, inclusive, for an electrode in the chargedstate. For the four preferred alloy compositions shown, x has a valuefrom 4 to 5 and B has a value from 0.1 to 1.0. Also shown in this tableare the atom percentages and weight percentages for lithium, silicon,and iron corresponding to the alloy compositions shown.

In Table 2 are summarized the overall composition range and thepreferred compositions, both in terms of weight percent and atompercent, for the ternary alloy. Referring to FIG. 1 of the drawing, theenclosed area in the composition diagram of FIG. 1A corresponds to theatom percent range for the ternary alloy compositions shown in Table 2,the preferred ternary alloy range being shown cross-hatched, andcorresponding to 70-80 Li, 15-20 Si, and 2-10 B, all in atom percent.Similarly, in FIG. 1B is shown the composition range in terms of weightpercent, the preferred composition range being shown cross-hatched andcorresponding to 42-56 Li, 42-48 Si, and 2-10 B, all in weight percent.It will be appreciated that the empirical formulas shown and the atomand weight percentages referred to herein refer to the lithium electrodein its formed or fully charged state, since obviously in operation ofthe cell the lithium will be discharged, resulting in an alloy ofsubstantially less or even no lithium content.

                  TABLE 1                                                         ______________________________________                                        Li-Si-B COMPOSITIONS                                                          Empirical                                                                              Atom Percent    Weight Percent                                       Formula  Li      Si      B     Li    Si    B                                  ______________________________________                                        LiSiB    40      40      20    17.3  69.3  13.4                               Li.sub.2 Si.sub.0.5 B.sub.1.32                                                         52.4    13.1    34.6  31.1  33.1  33.8                               Li.sub.2 SiB.sub.0.5                                                                   57.1    28.6    14.3  29.5  59.1  11.4                               Li.sub.4 SiB.sub.0.5                                                                   72.7    18.2    9.1   45.6  45.6  8.8                                Li.sub.5 SiB.sub.0.5                                                                   76.9    15.4    7.7   51.1  40.9  8.0                                Li.sub.5 SiB.sub.0.1                                                                   82.0    16.4    1.6   54.6  43.7  1.7                                ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        SUMMARY OF Li-Si-B COMPOSITION RANGES                                                Atom Percent  Weight Percent                                                  Li    Si      B       Li    Si    B                                    ______________________________________                                        Overall                                                                       Range:   30-85   12-40   3-30  12-61 35-68 4-20                               Preferred                                                                     Range:   70-80   15-20   2-10  42-56 42-48 2-10                               ______________________________________                                    

For certain special applications, depending particularly on thetemperature of use and the nature of the electrolyte with which theelectrode will be in contact, it is contemplated that the lithium alloyelectrode may be self-supporting. However, for most applications,particularly where the lithium alloy electrode is in contact with amolten salt electrolyte at elevated temperatures, the lithium-alloyelectrode structure further includes a supporting current-collectingmatrix in intimate contact with the alloy.

Suitable materials for the supporting current-collecting matrix arethose materials resistant to attack by lithium or lithium-silicon-boronmixtures. Examples of such materials include iron, steel, stainlesssteel, molybdenum, titanium, tantalum, and zirconium. The purpose ofproviding a matrix in intimate contact with the alloy is to provide forsubstantially uniform current density throughout the alloy and also toprovide structural support for the alloy. It has been determined thatthe lithium-silicon-boron alloy utilized in the present inventiongenerally lacks structural integrity when used in an electrical energystorage device as the sole component of the negative electrode,particularly in a molten salt electrolyte at its high operatingtemperature. To function for any significant length of time withoutdisintegration, therefore, it is preferable if not actually essentialthat the formed lithium alloy be provided with a supporting matrix. Itis contemplated and preferred, within the scope of this invention, thatthe support and current-collecting capability be provided by a singlestructure; however, the support may be provided by one structure and thecurrent-collecting capability by another separate structure.

The matrix may be in the form of an electronically conductive poroussubstrate having an apparent density of from about 10 to 30 percent ofthat of the base material. Advantageously, the substrate will have amedian pore size within the range of from about 20 to 500 microns andpreferably from about 50 to 200 microns. A preferred form of such asubstrate is formed from woven or non-woven wires pressed together to adesired apparent density and then sintered. Such pressed and sinteredwire structures are known and commercially available ad Feltmetals. Theporous substrate then is impregnated with the alloy, for example, byimmersion in a molten bath of the alloy followed by removal and cooling.Alternatively, the alloy may be cast about a matrix formed from a wirescreen or expanded metal.

In another variation, the matrix structure may be in the form of aperforate container formed from wire screen or the like, and containingtherein a body of the alloy alone. Alternatively, the alloy is inintimate contact with a porous substrate enclosed in the perforatecontainer, it being desirable that the container and the substrate be inelectrical contact with one another. This latter variation isparticularly useful when the porous substrate is formed from very finewoven or non-woven wires pressed together to form a body.

More particularly, it has been found, at least in the case of iron usedas the substrate material, that if the wire used to form the poroussubstrate has a diameter of less than about 10 microns, the substratetends to break up and disintegrate upon repeated charging anddischarging of the electrode in a molten salt electrolyte. It is notknown with certainty whether such destruction is the result ofimperceptible expansion and contraction of the electrode or theoccurrence of a chemical interaction. In selecting material for use as asubstrate, therefore, consideration should be given to any chemicalreaction or corrosion that may occur as a result of the specificelectrolyte or matrix material which is utilized. Further, if the matrixcomprises woven or non-woven wires passed together to provide a poroussubstrate, the wire should have a diameter of at least about 10 microns.Advantageously, the wire diameter will be from about 10 to about 500microns and preferably from about 10 to 200 microns.

A particularly suitable supporting current-collecting matrix electrodestructure that may be utilized in the present invention is shown in U.S.Pat. No. 4,003,753, assigned to the Assignee of the present invention,said patent being hereby incorporated by reference. Broadly, this matrixstructure comprises a unitary multi-cell structure including a pluralityof wall members having edges and axially extending surfaces which form aplurality of cells having at least one open end, said cells having across-sectional area of at least about 0.04 cm². The edges of the wallmembers in the open end of the cells are aligned in a common plane toform a planar face. Generally the electrode structure will have a planarface having a surface area of from about 25 to 300 cm². The axiallyextending surfaces of the wall members are substantially perpendicularto the planar face. The body of electrochemically active alloy materialis disposed in the cells, being retained in place by anelectrolyte-permeable member which is affixed to the wall members andcovers the open end of the cells. This type of matrix electrodestructure may also be utilized for containing a body ofelectrochemically active positive electrode material.

The multi-cell matrix structure is essentially a macroporous oropen-faced cellular structure. The individual cells may take variousforms, however, such as squares, diamond shapes, rectangular, circular,octagonal, or indeed just about any geometric shape. Further, theindividual cells may or may not share a common wall. The particularlypreferred form is one in which the individual cells are hexagonal inshape, sharing a common wall to form a honeycomb structure. Thispreferred shape optimizes the void volume for retention of activematerial while at the same time providing a high strength-to-weightratio. In some instances, however, other less complex forms such assquare-shaped cells may be preferred for economic reasons. An advantageof this matrix structure over the prior art porous matrix structures isthe ease with which it can be uniformly loaded with active materials.

The cell depth of the multi-cell structure is not particularly critical.Generally, it has been found that good utilization of theelectrochemically active positive or negative electrode material isattainable with cells having a depth of from about 0.1 to 2.0 cm andpreferably from about 0.5 to 1.0 cm. It will be appreciated, however,that the depth of the cell and thickness of the wall members of ofmulti-cell structure should be such as to provide structural integrityand resist warping. Particularly good results have been obtained withrespect to effective utilization of active material and structuralintegrity when the ratio of the open cross-sectional area of the cell tothe depth of the cell is maintained within a range of from about 1:1 to2:1 and the wall members of the cell have a thickness within the rangeof from about 0.002 to b 0.05 cm, preferably from about 0.002 to about0.02 cm.

The particular material selected for the electrode structure formed bythe method of the present invention is not critical except insofar as itmust be one which is not attacked or corroded by the molten electrolyteduring normal operation of the device. Generally, iron, steel, or nickelsteel alloys are preferred on the basis of cost for containing theLi-Si-B alloy material. Molybdenum, titanium, and tantalum are preferredon the basis of their corrosion resistance, however, the cost of thesemetals are generally prohibitive.

The electrolyte-permeable member may be conductive or non-conductive andfills two functions: (1) to permit free passage of charged ions andelectrolyte into and out of the cells, and (2) to retain the activematerial in the cell. It has been found that the structural integrity ofthis matrix electrode structure is greatly enhanced when theelectrolyte-permeable member is fixedly attached to the wall members,preferably at the edges of the wall members, for example, by welding,brazing, or diffusion bonding.

In a particularly preferred embodiment, the electrolyte-permeable memberis formed from a wire screen wherein the individual wires have adiameter of from about 0.002 to 0.02 cm, the opening in theelectrolyte-permeable member should have a cross-sectional area withinthe range of from about 1 × 10⁻⁶ to 1 × 10⁻³ cm², and there should beprovided from about 10⁵ to 10² openings per square centimeter. Theelectrolyte-permeable member preferably is made from the same materialas the wall members. In addition to screens, other forms which may beused are porous sintered plaques, perforate plates, and the like. Whilethe wire screen is applicable to both positive and negative electrodestructures because of its low cost, these other forms also may be used.When a porous plaque such as porous nickel, iron or the like is used, itshould have an apparent density of from about 20 to 60% of that of thebase metal and an average pore size of from about 1 to 20 microns.

In one embodiment of the invention, the electrode is formed bysurrounding the matrix with the alloy in a molten state, for example, byimmersing a porous substrate in a molten body of the ternary alloy. Thealloy may be formed by mixing particulate lithium, silicon, and iron andheating such a mixture to a sufficiently elevated temperature to form amelt. In accordance with a preferred method however, the lithium firstis heated, in an inert atmosphere, to a temperature above the meltingpoint of lithium, and thereafter a boron silicide, typically aborosilicon, is added in an amount to provide the desired weight percentfor the ternary alloy. In such latter method, the exothermic reactionbetween the lithium and the borosilicon will provide substantially allof the heat required to form a melt of the alloy.

It is particularly advantageous and preferred that the present lithiumalloy electrode be formed electrochemically in a molten salt electrolytein generally the same manner as known and utilized in forminglithium-aluminum and lithium-silicon electrodes. See, for example, U.S.Pat. No. 3,947,291. In the present invention, assembling a cell with atleast the negative electrode, and preferably both electrodes, in the"uncharged" state is particularly desirable because of the substantiallycomplete utilization of silicon obtained. Specifically, borosilicon inintimate contact with the supporting current-collecting matrix and amixture of lithium sulfide and iron as uncharged positive electrode areimmersed in a molten salt electrolyte containing a source of lithiumions, and the lithium is coulometrically charged into the electrode inan amount to form the desired alloy. At the same time iron sulfide isbeing formed as the positive electrode.

The boron silicides may contain minor amounts of impurities such as, forexample, calcium, magnesium, chromium, cerium, manganese, aluminum,carbon, and zirconium.

It is clear from a theoretical basis why boron is able to form asuitable ternary alloy with lithium, achieving high utilization ofsilicon, compared for example with molybdenum silicide which does notform a suitable lithium alloy. Although applicant does not wish to belimited in this regard to the following proffered explanation, it isbelieved that because of the weaker bonding between the boron andsilicon linkages there is a negative free energy of formation of thelithium compound, thereby promoting the reaction. The reduced migrationof silicon into the substrate or supporting matrix of the electrode isbelieved to be inhibited by a reaction between the boron, silicon andsubstrate metal which forms a protective boron-silicone-metal compoundcoating on the exposed surfaces of the substrate. The protection could,of course, be provided by depositing a film of boron on the substrateprior to loading the substrate with lithium and silicon. Reliablyobtaining a uniform coating or film of boron on the substrate isdifficult, however, particularly when the substrate has a complexconfiguration. Further, to be assured that all of the surface of thesubstrate is covered it is necessary to apply a relatively thick coatingof boron which results in an undesirable increase in resistance and costsince boron is expensive and in elemental form is a poor conductor.Accordingly it is preferred to provide the boron in the form of thepreviously described ternary alloy. Nonetheless a thin coating of boronon the substrate in addition to the ternary alloy is a desirable option.

An electrical energy storage device, particularly a second cell orbattery, includes the lithium electrode formed by the method of thepresent invention as the electrically regenerable negative electrode, apositive electrode, and an electrolyte.

The positive electrode or cathode is an electron acceptor and containsan active material which is electropositive with respect to the lithiumelectrode. The active material of the cathode may be sulfur or a metalhalide, sulfide, oxide or selenide. Suitable metals include copper,iron, tungsten, chromium, molybdenum, titanium, nickel, cobalt, andtantalum. The sulfides of iron are particularly preferred for use withmolten salt electrolytes. The cathode may be formed entirely of theactive material or may comprise a composite structure such as a holderof, for example, graphite containing a body of such active material, orthe multi-cell matrix electrode structure previously described.

The electrolyte utilized in preferred cell embodiments is alithium-ion-containing molten salt electrolyte; alternatively, forcertain particular cell systems, a solid electrolyte, an organic solventelectrolyte or an aqueous electrolyte is utilizable.

The term "molten salt electrolyte" as used herein refers to a lithiumhalide-containing salt which is maintained at a temperature above itsmelting point during operation of the electrical energy storage device.The molten salt may be either a single lithium halide, a mixture oflithium halides, or a eutectic mixture of one or more lithium halidesand other alkali metal or alkaline earth metal halides.

Typical examples of binary eutectic salts are lithium chloride-potassiumchloride, lithium chloride-magnesium chloride, lithium chloride-sodiumchloride, lithium bromide-potassium bromide, lithium fluoride-rubidiumfluoride, lithium iodide-potassium iodide, and mixtures thereof. Twopreferred binary salt eutectic mixtures are those of lithium chlorideand potassium chloride (melting point 352° C), and lithium bromide andrubidium bromide (melting point 278° C).

Examples of ternary eutectics useful as the molten salt electrolyteinclude calcium chloride-lithium chloride-potassium chloride, lithiumchloride-potassium chloride-barium chloride, calcium chloride-lithiumchloride-barium chloride, and lithium bromide-barium bromide-lithiumchloride. Preferred ternary eutectic mixtures include those containinglithium-chloride, lithium fluoride and lithium iodide (melting point341° C) and lithium chloride, lithium iodide and potassium iodide(melting point 260° C).

The suitable alkali or alkaline earth metal ion should have a depositionpotential very close to or preferably exceeding deposition potentials oflithium ion in the electrolyte. Lithium halide salts can be readilycombined with halides of potassium, barium, and strontium. Halides ofmetals such as cesium, rubidium, calcium, or sodium may be used, but asubstantial proportion of these metals may be co-deposited with thelithium when the electrode is charged, with a resulting small loss inpotential.

Although the ternary eutectic salt mixtures, particularly thosecontaining the iodides, provide lower melting points, the binaryeutectic mixture of lithium chloride-potassium chloride sometimes ispreferred on the basis of its lower cost and availability, particularlyfor batteries to be used in large scale applications such as electricpowered vehicles and electric utility bulk energy storage.

If desired, a lithium chalcogenide corresponding to the chalogenide ofthe positive electrode is added to the molten salt. Thus, when thepositive electrode material is a sulfide or oxide, Li₂ S or Li₂ O isadded, respectively, to the molten salt. It has been found that if asaturating amount of the lithium sulfide (about 0.1 wt.%) or lithiumoxide (about 0.4 wt.%) is added to the fusible salt electrolyte,long-term cell performance is enhanced.

The solid state electrolytes contemplated herein include a mixture oflithium sulfate and a lithium halide such as lithium chloride or lithiumbromide or a mixture thereof. The composition of the mixed salt solidelectrolyte may vary from 10 to 95 mole % lithium sulfate. Solidelectrolytes having such composition are conductive in what appears tobe a solid phase at temperatures as low as about 400° C.

The lithium electrode formed by the method of the present invention alsois useful in electrical energy storage devices, particularly primarycells, which utilize a lithium-ion source in an organic solvent. Theterm "organic electrolyte" contemplates those non-aqueous electrolyteswhich comprise an organic solvent and a solute. The solute is the sourceof lithium cations. The solute also is, of course, miscible or dissolvedin the organic solvent. The solvent is such that it does not attack theelectrode materials and is not affected by them. Obviously the soluteshould be stable in its environment at the intended operatingtemperature and electrical potential. Organic electrolyte cellsgenerally are designed to operate at a temperature below about 125° C,and more specifically, at a temperature within the range of from about0° to 80° C. It is important that the solute and the solvent be such asto provide a lithium ion-containing and conducting medium which ismobile or liquid under these conditions. Normally, it is preferred thatthe solute be of high purity.

The solutes which most nearly meet these requirements are lithium halidesalts. For conductivity purposes, other metal halides, e.g, aluminumchloride, are often complexed with the lithium halide. The halide isselected from the group consisting of chlorine, fluorine, bromine,iodine, and mixtures thereof. It is envisioned that double anioncomplexes also could be used. Examples of suitable solutes are lithiumbromide, lithium chloride, lithium fluoride, sodium chloride, sodiumfluoride, and potassium chloride. The lithium salt also may be a lithiumperchlorate, hexafluophosphate, tetrafluoborate, tetrachloroaluminate,or hexafluoarsenate.

Preferably, the lithium ion-containing and conducting medium used is ina saturated or supersaturated condition. The ion-containing andconducting medium should have sufficient salt concentration to permitmost economical operation of the cell. The ion-containing and conductingmedium should have a concentration of solutes greater than about 0.5molar.

The choice of organic solvent for the ion-containing and conductingmedium is dictated by many of the considerations involving the solute.The solvent of the ion-containing and conducting medium is any polarmaterial which meets the requirements of serving as a transfer mediumfor the solute and in which the solute is miscible or dissolved. Thesolvent also should be of such a material as to be inert to theelectrode materials. The solvent is preferably a liquid at from about0°-125° C; operating conditions dictate such a requirement. For example,dimethylsulfoxide is an excellent solvent above its melting point ofabout 18.5° C. The solvent is desirably one which does not readilyrelease hydrogen ions. Solvents of high dielectric constants and lowviscosity coefficients are preferred.

Suitable solvents are, for example, the nitriles such as acetonitrile,propionitrile, sulfoxides such as dimethyl-, diethyl-, ethylmethyl-, andbenzylmethylsulfoxide; pyrrolidones such as N-methylpyrrolidone, andcarbonates such as propylene carbonate.

The anodic reaction of alkali metals in aqueous electrolytes in anelectrochemical cell to produce electrical energy is also known. Ofparticular interest is a highenergy lithium-water primary cell, whichutilizes a lithium or lithium alloy anode, an inert cathode such asplatinum, nickel, or silver oxide, and an aqueous alkali metal hydroxideelectrolyte, such as sodium hydroxide or potassium hydroxide. Thelithium alloy electrode formed by the method of the present invention isconsidered as suitable for use as an anode in such a lithium-waterprimary cell system.

In addition to the foregoing representative list of suitableelectrolytes and positive electrode materials, many others will beapparent to those versed in the art. It is not intended that theinvention be limited, therefore, only to those specifically identified.

Referring now to FIG. 2 of the drawing, there is shown a graphicalcomparison of a typical charge-discharge curve for a Li-Si-B alloyelectrode. As may be noted from FIG. 2, the Li-Si-B electrode chargesand discharges through four distinct voltage plateaus, the final plateauof about 48 mv being below the potential of liquid lithium. This chargedischarge curve is substantially the same as that for a lithium-siliconalloy, thus, it is seen that the addition of boron has no deleteriouseffect on the electrical performance of the alloy.

The reason for such a series of different potentials withlithium-silicon or lithium-silicon-boron alloys is not known withcertainty, and the present invention is not to be considered as limitedby any particular theory. It is believed, however, that the differentplateaus represent specific species of lithium alloy compounds.Obviously, knowledge of the precise mechanism involved is not necessaryfor the practice of the present invention.

Referring to FIG. 2, it also will be noted that the alloy is formed to apotential which is below that of liquid lithium. At the liquid lithiumpotential, it is possible that the release of free lithium into theelectrolyte may occur. However, it should be noted that even where it isdesired to completely eliminate such a possibility by electroforming toa lower plateau than that of liquid lithium, in all instances thepercent utilization of silicon for the Li-Si-B alloy is usuallysignificantly greater at equivalent plateaus than for a Li-Si- alloy.Also, to guard against the possibility of free lithium being present,the Li-Si-B alloy having a lower lithium atom percent may be formed suchas that corresponding to Li₄ SiB₀.5 (atom % Li is 72.7) rather than Li₅SiB₀.1 (81.6 atom % Li).

Referring now to FIG. 3, a lithium electrode 10 formed by the method ofthe present invention is shown. The electrode 10 includes a conductorwire 12 and a cage or a perforate container matrix formed from a wirescreen 14 and a porous substrate impregnated with alithium-silicon-boron alloy 16. Alternatively, the multi-cell honeycombstructure shown in U.S. Pat. No. 4,003,753 may be used as the matrixstructure.

In FIG. 4 is depicted an electrical energy storage device 20 whichutilizes the lithium alloy electrode formed by the method of the presentinvention. The storage device 20 includes a positive electrode 22 and anegative electrode 24, the latter comprising a porous metal substrateimpregnated with a lithium-silicon-boron alloy. Electrodes 22 and 24 areprovided with electrical connectors 26 and 28, respectively. Theelectrical energy storage device also includes a housing 30 and a cover32. The cover 32 is provided with apertures therethrough for electricalconnectors 26 and 28. Located within the apertures are electricallynonconductive insulators 34. The electrical energy storage device alsoincludes an electrolyte 36. When the electrolyte is a solid electrolyteor a molten salt electrolyte, both of which must operate at relativelyhigh temperatures, housing 30 also may be provided with heating meanssuch as a plurality of electrical resistance heaters 38.

The following examples are set forth for the purpose of illustrating thepresent invention in greater detail, but are not to be considered aslimitations thereof. Thus the examples principally relate to use of theelectrode formed by the method of the present invention in a molten saltelectrolyte which is preferred. However, the electrode formed by themethod of the invention should not be construed as limited to electricalenergy storage devices utilizing only a molten salt electrolyte, for, asherein disclosed, it will also have utility in an electrical energystorage device utilizing either a solid electrolyte, an organicelectrolyte, or an aqueous electrolyte.

EXAMPLE 1

To demonstrate that the lithium-silicon-boron alloy formed by the methodof the present invention has utility as an electrode and further thatthe addition of the boron permits the forming of a negative electrodefrom a discharged state, the following tests were performed. Fiveelectrodes were assembled using a five square centimeter piece of lowcarbon steel honeycomb. The electrodes were individually tested byimmersing them in a lithium chloride-potassium chloride eutectic moltensalt electrolyte having a melting point of about 352° C. The electrodeswere then cycled against a lithium counter electrode. The results of thetest are set forth in Table 3. The utility of the electrodes formed bythe method of the presently claimed invention is seen in Table 3.Specifically, electrodes one and three were readily charged to in excessof 90% of the theoretical ampere-hour capacity. In addition, nosignificant polarization of the electrodes was observed until thecurrent density was increased to about 60 mA/cm² (electrode #5). Thusthis example clearly demonstrates the utility of the ternarylithium-silicon-boron alloy for use as a negative electrode in a moltensalt electrolyte.

                                      TABLE 3                                     __________________________________________________________________________                            Utilization                                                 Electrolyte                                                                         Theoretical                                                                         Current                                                                             Percent of                                            Electrode                                                                           Temperature                                                                         Capacity                                                                            Density                                                                             Theoretical                                                                         Cycled                                          No.   (° C)                                                                        (A hr/cc)                                                                           (mA/cm.sup.2)                                                                       Capacity                                                                            Between                                         __________________________________________________________________________     1*   425   1.95  10     96.9                                                                                ##STR1##                                       2     425   1.95  40    93                                                                                   ##STR2##                                        3*   425   1.0   40    99                                                                                   ##STR3##                                       4     366   0.8   20    82                                                                                   ##STR4##                                       5     366   0.8   60    57                                                                                   ##STR5##                                       __________________________________________________________________________     *Formed from uncharged state, i.e. SiB.sub.0.5                           

EXAMPLE 2

This example is set forth to demonstrate the decrease in siliconmigration into an electrode supporting structure when using alithium-boron-silicon alloy. One pair of test electrodes was made usingan AISI 1010 steel honeycomb for the supporting structure or matrix. Oneelectrode was filled with lithium-silicon-iron and the other withlithium-silicon-boron. The two electrodes were then cycled against alithium counter electrode for 340 hours at a temperature of 450° to 470°C. At the end of the test the electrodes were removed and subjected toanalysis to determine the extent of silicon migration into thesubstrate. The scale thickness (silicon migration depth) was two tothree times greater on the lithium-silicon-iron electrode than on thelithium-silicon-boron electrode. The substrate used for thelithium-silicon-boron alloy was found to have a scale containingsignificantly increased amounts of boron, thus demonstrating that theboron plays a significant role in preventing the formation of Fe₃ Si atthe substrate surface. When this test is repeated comparinglithium-silicon to the alloy formed by the method of the presentinvention, the silicon migration depth is 6 to 12 times greater on thelithium-silicon substrate.

A similar test was performed using a pair of electrodes which utilized astainless steel honeycomb as the supporting structure. In this test theelectrodes were cycled for 1500 hours at a temperature of 425° to 450°C. Following this test the electrodes were subjected to analysis forscale thickness (silicon migration). The electrode substrate which hadbeen filled with lithium-silicon-iron had a scale thickness 30 to 40%greater than that of the supporting structure which has been filled withthe alloy of the present invention. Thus these tests clearly demonstratethe efficacy of the alloy of the present invention for supressingsilicon migration into the supporting structure of an electrode.

As may be noted from the foregoing examples, the present invention isparticularly advantageous in offering the ability to assemble a completecell with both the negative and positive electrodes initially in theuncharged state and then electroforming in situ. Because of the lessersensitivity to oxygen and moisture of the uncharged electrodes in theabsence of lithium, ease of handling, charging, and fabrication isgreatly facilitated. By contrast, starting with an initially dischargedcondition and forming lithium-silicon negative electrodes byelectrochemically charging silicon powder is disadvantageous, since ithas been found difficult to utilize more than half of the silicon powderpresent in the electrode structure.

It will of course be realized that various modifications can be made inthe design and operation of the lithium electrode formed by the methodof the present invention without departing from the spirit thereof.Thus, while the formed lithium electrode structure has been illustratedand described with respect to certain exemplary embodiments relating toparticular preferred constructions and materials for the supportingcurrent-conducting matrix electrode structure, and while preferredembodiments of secondary cells utilizing molten salt electrolytes andmetal sulfide cathodes have been illustrated and described, it should beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically illustrated anddescribed.

What is claimed is:
 1. A method for electrochemically forming acomposite negative lithium electrode structure for an electrical energystorage device from the uncharged state, wherein the formed electrodestructure comprises a supporting current-collecting matrix in intimatecontact with an alloy of lithium, silicon, and boron having theempirical formula Li_(x) SiB_(y), where x has a value from 1 to 5 and yhas a value from 0.1 to 3, which includes the steps of:providing asupporting current-collecting matrix in intimate contact with an alloyof boron and silicon, the boron being present in an amount from about 11to 50 atom percent and the silicon being present in an amount from about89 to 50 atom percent; immersing the current-collecting matrixcontaining the boron-silicon alloy in a lithium-containing molten saltelectrolyte in a cell in opposing relation to a positive electrodestructure, and electrochemically charging the boron-silicon electrodestructure at a selected voltage or current for a time sufficient to formthe desired lithium-silicon-boron alloy negative electrode structure. 2.The method of claim 1 wherein the positive electrode prior to chargingcomprises a mixture of lithium sulfide and iron.
 3. The method of claim1 wherein x has a value from 4 to 5 and y has a value from 0.2 to 1 inthe formed negative electrode structure.